This invention pertains to a fluid pressure switch or device for detecting two or more distinct fluid pressure levels from a varied range of such levels; switches of this type are conventionally found in fluidic control systems. Typically, the detection of fluid pressure at various levels has been accomplished by installing a dedicated pressure switch for each fluid pressure level sought to be detected. Therefore, not only a large space was required in the switch, but also the installation and the maintenance of the switches were complicated and troublesome; moreover, the cost of the whole device was high.
Consequently, these problems led us to the invention of new fluid pressure switches which can detect fluid pressure levels at more than two points. The assignee of this patent application has applied for a patent for the fluid pressure switch based on FIG. 8 (Japanese patent application: 1978 No. 154419).
The fluid pressure switch, as shown in FIG. 8, has a piston 102 installed on the stepped hole 101 in the body 100, and one end of the piston is designed to be operated by the detecting fluid pressure P. Contacting the other end of the piston 102, there is a piston rod 104 through the upper end 103 of the body 100.
When the piston rod 104 moves vertically with the piston 102, two switches, 105 and 106, open and close in order. There is a spring 108 installed between the contacting side 107 of the piston rod 104 and the upper end 103 of the body 100 in order to operate the piston 102 to resist the detecting fluid pressure P. Another spring 111 is also installed between the upper end 103 of the body 100 and the spring receiver 110 which is connected to the landing 109. Therefore, when the piston 102 moves and contacts the spring seat 110 to resist the force of the spring 108, the force of the other spring 111 is added to the piston 102.
A fluid pressure switch structured in this way has piston stroke characteristics as shown in FIG. 9. When the detecting fluid pressure P operates one side of the piston 102, the piston 102 and the piston rod 104 rise, resisting the force of the spring 108 with increasing pressure in the pressure area A in FIG. 9, and the detecting fluid pressure P reaches the pressure P.sub.1, the piston rod 104 makes the switch 105 operate, and the pressure P.sub.1 is detected. When the pressure P increases up to the pressure P.sub.x, the piston 102 contacts the spring receiver 110, the force of the other spring 111 is added to it, and the joint force of the two springs 108 and 111 becomes greater than the fluid pressure P.sub.x, so the piston 102 and the piston rod 104 stop further upward motion. This motionless state continues through the pressure area B, until the fluid pressure P reaches P.sub.y which can overcome the joint spring force. After that, the fluid pressure P enters the pressure area C, passing the pressure P.sub.y, the piston 102 and the piston rod 104 rise again, because the fluid pressure P becomes greater than the joint spring force, and when the fluid pressure P reaches P.sub.2, the piston rod 104 activates another switch 106 and P.sub.2 is detected.
A fluid pressure switch having the feature described above, has the following merits. The volume of the piston stroke or simply, piston stroke, can be made smaller, even if there is a big difference between detecting pressure levels because the piston 102 and the piston rod 104 stop their motion through the pressure area B, so the whole device can be made to a smaller scale. Another feature is that it has a good detecting performance for the pressure P.sub.1, due to the possibility of taking a larger gradient in the low pressure area A.
Despite these merits, however, the following problems occurred. One of such problems is that fluid pressure in the area B cannot be detected, and detecting pressure levels cannot be set in the area B. This problem can be solved by changing the pressure property of the piston strokes in the area B and the area C into the dotted chain line as shown in FIG. 9, but this causes a new problem, i.e., it makes the structure of the device more complicated because it requires the installation of an adjusting system in order to eliminate the area B.
Another problem is the lack of flexibility in designing springs. It is necessary to have precision detection, especially in the low pressure area A. In order to achieve it, the spring constant of the spring 108 for the low pressure area should be made smaller and it is necessary to make the gradient of the piston strokes in the pressure area A, but the spring 108 is compressed not only in the low pressure area A but also in the high pressure area C. Therefore, the strength of the spring for the period of high pressure input has to be considered.
When it comes to the design of the springs, there are many required conditions: the spring constant has to be small; the spring must have sufficient strength to resist the compression; and the size of the spring must be small enough that it does not take a large storage space.
Those are the main points of the problems, but there lies another problem relating to processing precision; detecting performance decreases in the pressure areas right before the pressure reaches P.sub.x or right after the pressure passes P.sub.y.